The use of polymers as viscosity modifiers (or viscosity index improvers) or dispersant viscosity modifiers in an oil of lubricating viscosity is well known. Typically polymer backbones include polymethacrylates, polyolefins or hydrogenated styrene-butadienes and functional derivatives thereof. For dispersant viscosity modifiers, the backbone may be functionalised with a grafted nitrogen compound.
Many of the known viscosity modifiers and dispersant viscosity modifiers have limited cleanliness performance and limited low temperature properties when the polymers degrade. This is often observed in engine piston deposits, particularly for olefin copolymers. Further, a number of viscosity modifiers and dispersant viscosity modifiers have limited low temperature performance.
Specifically for engine lubricants, limited cleanliness performance may result in ring stick or accumulation of deposits on valves or the tract. Deposit accumulation in valves may result in misting, air intake debris and valve stem seal leakage and cause other engine operation problems.
It is also further known that additives with long chain alkyl groups, such as polyisobutylene (commonly from succinimide dispersants), have a detrimental impact on fuel economy and cold crank. Therefore it would be a further advantage to reduce the amount of additives with a detrimental impact of fuel economy from engine oils.
In engine lubricants for marine diesel applications, due to limited supply of bright-stock, there is a need for an alternative material capable of providing a viscosity-modifying boost. Therefore, it would be a further advantage to identify a viscosity modifier capable of replacing brightstock.
U.S. Pat. No. 5,512,192 discloses a dispersant viscosity improver for lubricating oil compositions prepared by the reaction of an oil soluble, substantially hydrogenated, vinyl substituted aromatic-aliphatic conjugated diene block copolymer, grafted with an ethylenically unsaturated carboxylic acid or functional derivative thereof. The dispersant viscosity improver typically has a number average molecular weight of 30,000 to 300,000; and a weight average molecular weight of 50,000 to 500,000.
U.S. Pat. No. 5,429,758 discloses graft copolymers prepared from solvent free reactions and dispersant derivatives thereof. The graft copolymers include a hydrogenated random or normal block copolymer made from a vinyl substituted aromatic monomer and a conjugated diene. The normal and random block copolymer has a number average molecular weight from 10,000 to 500,000.
US Patent Application 2005/0153849 discloses grafting a polymer backbone selected from olefin polymers, diene polymers, vinyl polymers and vinylidene polymers, which have been further reacted with a variety of amines.
The present invention provides a copolymer capable of being used as viscosity modifiers and/or dispersant viscosity modifiers capable of providing at least one of acceptable low temperature performance and/or cleanliness. When the lubricant composition is suitable for engine oils, the invention further provides at least one of acceptable fuel economy, whilst maintaining cleanliness, and soot and sludge handling.